Various techniques exist for driving inductive loads. One popular method is the "full bridge" configuration (also called the "H" bridge configuration) as shown in FIG. 1. Semiconductor switches 50, 52, 54 and 56 are coupled to a load 58 such that the circuit looks like the letter "H". This method is popular because it allows the load to be driven in both directions (through conduction path I and conduction path II). This enables one to drive bi-directional motors, etc.
The "H" bridge drive configuration, however, does have disadvantages. After switching, inductive flyback can forward bias transistor junctions that are designed to remain reverse biased, causing electrons to be injected into the substrate which in turn cause parametric shifts and potential latch-up conditions leading to device failure. (Inductive flyback is an abrupt change in voltage across the terminals of an inductive load caused by the load attempting to change instantaneously; this property is well known and understood by those skilled in the art.) The substrate current injection phenomena can be clearly seen in FIG. 2. FIG. 2 is a cross section drawing of a prior art LDMOS (lateral double diffused MOS) transistor 60. Transistor 60 has a metal source contact 62 making electrical contact to both N+ source contact 68a and P+ backgate contact 70a which electrically shorts source diffusion 68b and backgate diffusion 70b together. Thus, a P-N junction exists between the backgate 70b and drain 72b. Therefore, if the voltage at source contact 62 to rise approximately 0.5 V above the voltage at drain contact 66 the P-N junction would become forward biased and current would be injected into the substrate.
Different methods exist to eliminate substrate injection. Placing external, discrete Schottky diodes in parallel with the output transistors shunt the injected substrate current away from the semiconductor die. This solution takes up space on a circuit board and increases the overall system cost. Surrounding the output transistor structures with guardrings in the layout to collect the substrate current requires backside metallization to minimize substrate debiasing and is, therefore, cost prohibitive. Further methods are needed to control substrate current injection.
It is an object of this invention to provide a novel "H" bridge drive circuit that controls inductive flyback voltage, thereby eliminating substrate injection. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.